AN: Sorry this was delayed. Much the same reason as the story proper...laptop issues. It's not that long, and I'm sorry for that. But there isn't much to go off of with the Forerunner. In any case, I'll explain my reasoning at the bottom as always.
UNSC/SGC Codex
Compiled by Dr. Catherine Halsey and Colonel Samantha Carter.
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Codex Entry Three: Forerunner
Weapon Systems:
Hardlight: The most prolific Forerunner weapons are the Sentinel beam and its larger counterparts on the Keyship Ecumene. They appear to function by using hardlight 'projectiles' and beams. These weapons are devastatingly effective, ripping through Covenant shields like butter. The weapons are hard to produce however, limiting any deployment by UNSC forces.
Needle Cannon: Mounted on larger Sentinel models, this appears to be a scaled up version of the Covenant Needler/Needle Rifle. The NC fires the same projectiles as its smaller cousins, but with devastating effectiveness. This weapon is fully capable of destroying a Wraith tank in a few direct hits.
Halo Array: Easily the most dangerous of Forerunner weaponry. The Seven Halo Rings are each capable of wiping out all sentient life within a 25,000 light year radius of the Installations. If all seven were to be fired at once, it would mean the end of all galactic civilizations, as there is no known defense against the power of a Halo.
Defense Systems:
Energy Shield: Broadly similar to the Covenant shielding systems, Forerunner shields are Type 2. As Covenant shields are reverse-engineered from Forerunner examples, we can extrapolate that the older shields are similar in function. They far outstrip anything the Covenant has in power however. A Forerunner Keyship can take more fire than even the much larger Supercarrier.
Power Supply: Placed in this category for the effect it has on the strength of the shields. Forerunner energy systems are highly advanced. Pound for pound, they are nearly as powerful as Ancient power systems, and more advanced than the Asgard's equivalent. A single power-room on a Halo Installation can put out more power than 2-3 ZPM's.
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End Entry Three
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Redirect to Entry Four: Energy Shields
AN: And here we go:
First...the weapons. I don't think anyone will argue with me on how effective they are. By all accounts, Forerunner ship-based weapons should make mincemeat of Covenant ships.
Second: Forerunner power supplies really are that effective. Just think about the amount of power needed to supply a death machine that can obliterate all sentient life within 25 thousand light years. I'm sorry, but it has to take several power rooms, each with the power of several ZPM's. The closest thing we have in Stargate is the Dakara Device, which needs the Stargates to spread its effect outside of Dakara itself.